The described embodiments relate generally to a battery pack system. More particularly, the present embodiments relate to improvements in packaging lithium-ion battery packs used in electronic devices to reduce the battery pack size and to improve system performance.
Recent advances in portable computing have utilized lithium-ion batteries to provide power to laptops computers, portable media players, personal digital assistants (PDAs), cell phones, tablets and other electronic equipment. Lithium-ion batteries are favored because, pound for pound, they are some of the most energetic rechargeable batteries available. They have a number of important advantages over competing technologies. They are generally much lighter than other types of rechargeable batteries of the same size because the electrodes of a lithium-ion battery are made of lightweight lithium and carbon.
In addition to consumer electronics, lithium-ion batteries are also popular for use in military, electric vehicle and aerospace applications. Lithium-ion batteries may be used in place of lead acid batteries in golf carts and utility vehicles. Lithium-ion batteries provide similar voltage levels as lead acid batteries so no modification of the vehicle's drive system is needed. The batteries could also be used for electric tools, medical equipment and other uses.
Lithium is a highly reactive element, meaning that a lot of energy can be stored in its atomic bonds. This translates into a very high energy density for lithium-ion batteries. For example, a typical lithium-ion battery can store 150 watt-hours of electricity in 1 kilogram of battery. A NiMH (nickel-metal hydride) battery pack can store perhaps 100 watt-hours per kilogram, although 60 to 70 watt-hours might be more typical. A lead-acid battery can store only 25 watt-hours per kilogram. Using lead-acid technology, it takes 6 kilograms to store the same amount of energy that a 1 kilogram lithium-ion battery can handle. For lightweight electronic devices, this is a significant advantage.
In addition to being lightweight, a lithium-ion battery pack may lose only about 5 percent of its charge per month, compared to a 20 percent loss per month for NiMH batteries. Lithium-ion battery packs have no memory effect, which means that a user does not have to completely discharge them before recharging, as with some other battery types. Lithium-ion batteries can also be recharged hundreds of times while other battery types may have more limited useful lives.
While lithium-ion batteries have many positive advantages, there are some disadvantages also. For example, lithium-ion battery packs require that a protection circuit be included to maintain voltage and current within safe limits. It is possible that failure of the battery pack due to overheating could cause harm to the accompanying electronic device, possibly even melting the device housing in certain instances. In order to avoid such situations, a protection circuit is generally included in a lithium-ion battery pack. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge may be greatly reduced.
However, while the protection circuits are included for safety reasons they tend to make the battery packs relatively more expensive and the inclusion of protection circuits make the battery pack larger than it would otherwise be. This increase in size may ameliorate some of the advantages discussed above as electronic devices become increasingly smaller and more compact.
A jelly roll design is the design used in many rechargeable batteries, and often for those batteries used in portable electronic devices. In this design, an insulating sheet is laid down, then a thin layer of an anode material is laid down, a separator layer is applied, and a cathode material is layered on top. This sandwich is then rolled up and inserted into a hollow casing. The battery, once wrapped, may be sealed in a flexible container. The container may have a tail or seam where the flexible material seals to itself in order to encompass the battery. Electrodes may extend through the tail in order to provide an electrical connection between the battery and internal components. The electrodes may exit the flexible container, bend, and be electrically connected to various components as desired.
Therefore, it would be desirable to have a battery pack for an electronic device which is more compact, while still including the safety features associated with a protection circuit built into the battery pack.